1. Field of the Invention
This invention relates generally to friction stir welding (FSW) of a hollow and spherical object. Specifically, the invention relates to the problems of creating a hollow metallic sphere with a reinforcing bisecting disk disposed therein, wherein toe sphere has a desired specific gravity that enables the sphere to have a neutral or positive buoyancy in a given fluid media.
2. Background of the Problems being Solved
Friction stir welding is a technology that has been developed for welding metals and metal alloys. The FSW process often involves engaging the material of two adjoining workpieces on either side of a joint by a rotating stir pin. Force is exerted to urge the pin and the workpieces together and frictional heating caused by the interaction between the pin, shoulder and the workpieces results in plasticization of the material on either side of the joint. The pin and shoulder combination or “FSW tip” is traversed along the joint, plasticizing material as it advances, and the plasticized material left in the wake of the advancing FSW tip cools to form a joint. The FSW tip can also be a tool without a pin, which is a shoulder that is still capable of processing another material through friction stir processing (FSP).
FIG. 1 is a perspective view of a tool being used for FSW that is characterized by a generally cylindrical tool 10 having a shank 8, a shoulder 12 and a pin 14 extending outward from the shoulder. The pin 14 is rotated against a workpiece 16 until sufficient heat is generated, at which point the pin of the tool is plunged into the plasticized workpiece material. Typically, the pin 14 is plunged into the workpiece 16 until reaching the shoulder 12 which prevents further penetration into the workpiece. The workpiece 16 is often two sheets or plates of material that are butted together at a joint line 18, or overlapping to form a lap joint. In this example, the pin 14 is plunged into the workpiece 16 at the joint line 18.
As the tool 10 is rotated, torque is transmitted from the rotating shank 8 to the FSW tip 24. When the tool 10 is being used on a workpiece that is a high melting temperature material such as steel, the FSW tip 24 is in many situations exposed to temperatures in excess of 1000 degrees C. as it is rotated while traversing steel softened by frictional heating.
Referring to FIG. 1, the frictional heat caused by rotational motion of the pin 14 against the workpiece material 16 causes the workpiece material to soften without reaching a melting point. The tool 10 is moved transversely along the joint line 18, thereby joining the workpieces as the plasticized material flows around the pin 14 from a leading edge to a trailing edge. The result is a solid phase bond 20 at the joint line 18 that may be generally indistinguishable from the workpiece material 16 itself, in contrast to welds using other conventional technologies. It is also possible that the solid phase bond 20 is superior to the original workpiece material 16 because of the mixing that occurs. Furthermore, if the workpiece material is comprised of different materials, the resulting mixed material may also be superior to either of the two original workpiece materials.
It is observed that when the shoulder 12 contacts the surface of the workpieces, its rotation creates additional frictional heat that plasticizes a larger cylindrical column of material around the inserted pin 14. The shoulder 12 provides a forging force that contains the upward metal flow caused by the rotating tool pin 14.
During friction stir welding, the area to be joined and the tool 10 are moved relative to each other such that the tool traverses a desired length of the joint. Depending upon characteristics of the tool 10, the tool may penetrate fully or partially into the joint. The rotating friction stir welding tool 10 provides a continual hot working action, plasticizing metal within a narrow zone as it moves transversely along the workpiece materials 16, while transporting metal from the leading edge of the pin 14 to its trailing edge. As a joint cools, there is typically no solidification as no liquid is created as the tool 10 passes. It is often the case, but not always, that the resulting joint is a defect-free, recrystallized, fine grain microstructure formed in the area of the joint.
Travel speeds are typically 10 to 500 mm/min with rotation rates of 200 to 2000 rpm. Temperatures reached are usually close to, but below, solidus temperatures. Friction stir welding parameters are a function of a material's thermal properties, high temperature flow stress and penetration depth.
Friction stir welding has several advantages over fusion welding because 1) there is no filler metal, 2) the process can be fully automated requiring a relatively low operator skill level, 3) the energy input is efficient as all heating occurs at the tool/workpiece interface, 4) minimum post-weld inspection is required due to the solid state nature and extreme repeatability of FSW, 5) FSW is tolerant to interface gaps and as such little pre-weld preparation is required, 6) there is typically no weld spatter to remove, 7) the post-joining surface finish can be exceptionally smooth with very little to no flash, 8) there is often no porosity and oxygen contamination, 9) there is little or no distortion of surrounding material, 10) no operator protection is required as there are no harmful emissions, and 11) joint properties are often improved. Throughout this document, friction stir welding will be considered to include all processes that can be performed using a friction stir welding tool, including but not limited to friction stir processing, friction stir spot welding and friction stir mixing.
Previous patent documents have taught the benefits of being able to perform friction stir welding with materials that were previously considered to be functionally unweldable. Some of these materials are non-fusion weldable, or just difficult to weld at all. These materials include, for example, 7075 aluminum, a material which is considered to be unweldable.
The previous patents teach that a tool for friction stir welding of high temperature materials is made of a material or materials that have a higher melting temperature than the material being joined friction stir welding. In some embodiments, a superabrasive was used in the tool, sometimes as a coating.
The embodiments of the present invention are generally concerned with these functionally unweldable materials, whether they are high melting temperature or low melting temperature but functionally unweldable.
Today's industry requires many structural components to perform a variety of functions and services in specific applications. Some of these applications require hollow structures made of a continuous material or materials. For example, a hollow sphere or cylinder would be such a structure. Practical applications using these hollow geometries would be pressure vessels and/or check valves used to either contain pressure on the inside of said structure, or withstand pressure from the outside or even some combination of the two. These applications require specific strength to weight ratios, specific gravities, uniform elastic properties, uniform plastic deformation properties, yield strengths, tensile strengths, as well as specific mechanical properties at varying temperatures and environmental conditions. Many of these applications require corrosion resistance and/or thermal stability as they are subject to extreme conditions.
Hollow structures such as pressure vessels, check valves and many other types of structures can be fabricated using the following five approaches:
Mechanical Attachment—Curved and/or flat components are assembled together according to design specifications and mechanically attached using screws, bolts or other mechanical fasteners to create an enclosed structure. Many times seals are used with these structures to ensure pressure specifications are maintained.
Welding—Conventional welding methods are used to join curved and/or flat components together to form a hollow structure. These methods include but are not limited to TIG, MIG, SubArc, ERW, Laser, etc.
Brazing—This technique is similar to welding but uses a lower melting temperature metal to join higher temperature components together.
Inertia welding—This technique is a solid state welding method that can be used to join circular components together.
Linear friction welding—This technique is similar to inertia welding except that the motion is linear as opposed to orbital.
Clearly, hollow structures can also be made by combining the above methods to achieve some success in order to construct the hollow structures.
3. Problems with Existing Art
Mechanical attachment using screws, bolts, fasteners, seals, etc. is disadvantageous because of high fabrication costs, increased weight, corrosion issues due to galvanic coupling, and design limitations. These costs arise from designing multiple components with accompanying tolerances, overdesign using safety factors in consideration of the stress risers created by mechanical attachment points, increased fabrication time and costs to meet tolerances, and fixturing costs to build components. In addition, in many cases, mechanical attachment to form an enclosed structural component cannot meet design requirements. In the case of pressure vessels, there is always rework to consider when pressure tests reveal leaks or excessive component strain.
Mechanical joints in hollow structures are also subject to fatigue failure at attachment points and stress riser locations and therefore require either more material, adding weight, or increased engineering expense to analyze structural performance with subsequent design modification for a given application. In addition, seals are used to contain pressure when components are fastened together and are the weakest material in the system and subject to leaks because of fit up, wear, thermal cycling damage, or inherent low strength.
Joining components using conventional welding methods has several severe disadvantages. Component materials must be selected based on weldability. Higher strength materials could lower the component construction cost, reduce the cost to the end user, reduce weight, and improve performance. However, many of the higher strength materials are not considered weldable. Further, the weld joint itself reduces the strength of the base metal components due to the weld's extensive heat affected zone and the cast microstructure created by melting and subsequent solidification. Following conventional fusion welding, base metal properties can be reduced by as much as 50%. More material must be used in the design of the structure to account for this weld zone weakness and therefore increases the weight and cost of the structure.
Because any fusion welding relies on melting of the faying surfaces, the weld is prone to solidification cracking, solidification defects, porosity, unpredictable and high residual stresses, material segregation, and component distortion. In addition, when a filler metal is required, the use of a dissimilar welding material increases the risk of galvanic corrosion between the weld and the base metal. Rework costs are extremely high to address distortion and additional welding attempts to repair cracks and welding defects.
Almost all of the nondestructive test methods that have been developed and are in use today were designed to identify poor quality weld joints. In fact, these tests are quite effective at locating and measuring any defect. The welding industry specifies that a crack is only a defect when it exceeds a given length even though it can be measured ultrasonically or with X-Ray. This acknowledges that welding inherently creates cracks and fusion welding is not capable of producing crack free structures. Design engineers attempt to account for these small cracks in structures by increasing component size or by overdesign to account for the inadequacies of welding.
Brazing has limitations different from those of welding. First, the braze alloy is a lower temperature material and thus is most often low strength, especially at elevated temperatures. Thus, brazing creates a weak location in the fabricated structure. Second, brazing requires the entire structure to be brought to elevated temperature. Often this is not practical for both cost and facility size limitations. Also, for many materials, the strength decreases at elevated temperature and is not recovered without costly subsequent heat treatments or, for work hardenable materials, cannot be recovered. Thus, for brazing, either material selection is limited or strength of the entire structure is reduced.
When higher strength materials cannot be used to create a hollow structure, with either mechanical attachment or welding because they fail to meet design requirements, a lower strength, lower density material must be used. By using lower strength materials, the overall design of the system using the solid structure must be downgraded, thereby limiting the design range of an entire system.
Some applications require the hollow structure to maintain a neutral or positive buoyancy in a given fluid media and would, for example, benefit from a hollow structure made from higher strength materials to resist higher forces. Since there is no method to make such a structure, a lower strength material with a given specific gravity is used and therefore limits the application's range of usefulness.
An example of a buoyant but lower strength material would be with a ball and seat in a check valve. Many check valves are designed to have a ball that releases from the seat when the pressure that seated the ball is removed. The ball must be buoyant in the fluid to float away from the seat and open the valve. The pressure of the entire system will be limited by the strength of the ball and seat material strength as well as the design. A lower strength solid ball with a given specific gravity could be used by increasing the size of the seat to minimize ball/seat contract stresses. The seat area will need to be substantially higher to accommodate higher operating fluid pressure. This can only be done by choking the system flow to account for reduced valve area. This greatly limits possible applications that require high fluid flow rates at high pressures.
Inertia welding can be used to join two hollow sections of a ball together by spinning one component against another. However, inertia welding limited to symmetric shapes, limited to certain materials, and the final shape is difficult to control due to the final upset procedure. Further, this joining method creates a tremendous amount of flash that forms both on the outside and inside of the component. Flash cannot be removed on the inside and is a variable that prevents careful control of the specific gravity of the component. This process is limited to smaller symmetrical parts and can produce heat affected zones similar to traditional fusion methods. Linear friction welding is also limited to specific shapes but also suffers from very high capital equipment costs and lack of available facilities to perform this very special joining method.
It would be an advantage over the prior art to provide a high strength material that can be used to create a hollow structure having a desired specific gravity and which can be manufactured to very precise dimensions of various diameters.